The present invention relates generally to solid state ionic devices and more particularly to multilayer, thin film solid state ionic devices which may be used as electrochromic windows and/or as rechargeable batteries.
Approximately 40% of the annual national energy consumption is used to control the climate of building interiors, i.e. to heat building interiors in the cooler months and to cool building interiors in the warmer months. Of this amount, approximately 33% is wasted, primarly due to radiation loss through building windows. For example, radiation loss occurs on warm days as solar energy is transmitted into the building interior through windows, causing the interior to be warmed and, consequently, requiring additional energy to be expended to cool the building interior. Additionally, radiation loss occurs on cool days as thermal infrared radiation present within the building interior escapes through building windows, thereby requiring additional energy to be expended as heat to warm the building interior.
One proposed solution to the problem of radiation loss has involved the use of electrochromic windows having variable absorption. The idea behind such windows is that, by absorbing solar energy, the windows prevent solar energy from entering the building interior and, therefore, from heating the building interior. Unfortunately, however, such windows frequently become very hot as a result of absorbing solar energy. Moreover, the windows ultimately re-radiate approximately 50% of the absorbed energy into or out of the building interior, resulting in thermal transfer inefficiencies.
A second proposed solution to the problem of radiation loss has involved the use of electrochromic windows having variable reflectance over a broad bandwidth of radiation. Using such windows, it is possible, for example, on warm days to transmit the visible portion of the solar spectrum so as to illuminate the building interior while reflecting the ultraviolet and infrared components of the solar spectrum so as to decrease the cooling load and, additionally, on cool days to transmit into the building interior the entire solar spectrum so as to both illuminate and heat the building interior while reflecting back into the building interior thermal infrared radiation already generated therewithin. Examples of variable reflection electrochromic windows are described in U.S. Pat. Nos. 4,889,414, 4,832,463, and 4,876,628, all of which are incorporated herein by reference.
According to the teachings of the aforementioned patents, a variable reflection electrochromic window typically comprises a transparent substrate and a thin film, multilayer coating whose transmissivity is adjustable by the transport of electrons and ions therethrough. Typically, the coating includes five layers, the first layer being a transparent electronically conductive layer which is deposited on the substrate, the second layer being an electrochromic layer whose spectral selectivity is adjustable, the third layer being an ion-conductive, electron-resistive layer capable of reversibly transporting positive metal ions into and out of the electrochromic layer so as to transform said electrochromic layer to and from an optically reflective state, the fourth layer being a counter-electrode layer capable of donating and receiving electrons and ions to and from said electrochromic layer, and the fifth layer being a transparent electronically conductive layer. As can readily be recognized, the first and fifth layers function merely as transparent electrical contacts for dispersing electrons over the surfaces of the second and fourth layers, respectively, and can be omitted from the device if desired.
In a preferred arrangement, the first and fifth (i.e. electron conductor) layers are formed from indium tin oxide; the second (i.e. electrochromic) layer is formed from WO.sub.3 ; the third (i.e. ion conductor) layer is formed from Li.sub.2 O:Nb.sub.2 O.sub.5 ; and the fourth (i.e. counter-electrode) layer is formed from LiCoO.sub.2. When an externally-generated electric field of the proper polarity is applied to the multilayered structure, lithium ions migrate from the LiCoO.sub.2 layer to the WO.sub.3 layer and become incorporated into the polycrystalline structure of WO.sub.3. The incorporation of lithium ions into the polycrystalline structure of WO.sub.3 causes that layer to become "colored," i.e., reflective to certain wavelengths of radiation. In a complementary fashion, the removal of lithium ions from LiCoO.sub.2 causes that layer also to become "colored."
Typically, all five of the above-described layers are deposited by a sputtering technique, such as by rf diode sputtering. However, the quality of some of those layers formed by sputtering is frequently less than desired. For example, as pointed out in U.S. Pat. Nos. 4,876,628 and 4,832,463, one of the difficulties in rf diode sputtering a mixed phase oxide resulting in stoichiometric Li.sub.2 O:Nb.sub.2 O.sub.5 for the third layer is that, when stoichiometric quantities of Li.sub.2 O and Nb.sub.2 O.sub.5 are prepared for rf sputtering onto the electrochromic layer, the resulting mixed phase oxides of lithium and niobium are less than stoichiometric and hence do not exhibit the optimum ionic conductivity and maximum electronic resistivity, as required within electrochromic layered structures.
One method for compensating for the loss of lithium ions during deposition of the third layer as disclosed in the aforementioned patents involves inserting lithium ions into the second layer via plasma injection before application of the third layer. As can be appreciated, however, this method itself creates a problem since the deposited lithium ions incorporate themselves in great numbers into the WO.sub.3 layer, causing that layer to be residually colored even when the electrochromic window has been switched so as to be in a transparent or "bleached" state.
In addition to causing the problems discussed above in connection with the deposit of the lithium niobate layer, sputtering techniques also typically result in a less than stoichoimetric deposit of the LiCoO.sub.2 layer, i.e., the composition of the resultant layer is Li.sub.x CoO.sub.2 wherein x is less than 1.0. Because the LiCoO.sub.2 layer is thus only partially lithiated, its spectral transmissivity cannot be fully modulated, i.e., it cannot be switched completely to a transparent or "bleached" state. Moreover, no method for altering the stoichiometry of the Li.sub.x CoO.sub.2 layer (i.e., increasing the value of x), once it has been thus deposited, has heretofore been known.
As can readily be appreciated, the above described electrochromic window can also be used as a rechargeable battery with the LiCoO.sub.2 layer serving as the cathode and the electrochromic layer serving as the anode. (It should be recognized that, in battery applications, the transparency of the substrate and the first and fifth layers as well as the electrochromicity of the second layer is unnecessary.) As discussed above, because the LiCoO.sub.2 layer of such a battery is lithium deficient, less charge can be transferred during its charging or discharging, thereby diminishing its usefulness as a battery.